1. Field of the Invention
This invention relates to aqueous slurries of pigmentary metal oxides, such as titanium dioxide. Reference hereafter to slurries is to such aqueous slurries.
2. Brief Description of the Prior Art
Physically stable high solids concentration pigmentary metal oxide slurries suitable for direct incorporation into aqueous paint or paper media represent the possibility of large cost savings to the paint or paper manufacturer compared with the use of dry pigmentary metal oxide powder which is often difficult to disperse in the paint or paper medium in question. Such slurries, however, are subject to the exacting requirement of remaining relatively stable during transport from the producer to the user even under fluctuating temperature conditions which may include below zero centigrade conditions and during what may be a relatively prolonged period of storage either by the producer or the user, but neverless being sufficiently flowable for delivery when required for use. The slurries are also subject to the requirement of giving acceptable pigmentary properties, preferably fully comparable with those obtainable when dry finished pigment is used directly. One example of an important pigmentary property is the gloss value of aqueous emulsion "vinyl gloss" paints containing the slurries. It is highly desirable for the slurries to be in gel form, preferably thixotropic gel form. This assists in preventing sedimentation under conditions of no, or low, shear while allowing the slurry to be delivered readily after the application of shear by mixing or, even, by movement under its own weight.
It is possible to prepare high concentration pigment slurries from previously dried and milled pigment. The drying and milling of a pigment requires the input of a very large amount of energy if satisfactory properties are to be achieved and it would be of considerable economic advantage to the pigment producer if pigment slurries having a satisfactory combination of properties to enable it to be transported and stored while retaining its pigmentary properties such as gloss in use were to be prepared directly from damp pigment resulting from the conventionally used surface treatment with inorganic oxides or, otherwise, from undried pigment.